An electrolytic copper foil is generally produced as follows. A rotating metal cathode drum with a polished surface is used along with an insoluble metal anode that surrounds said cathode drum and is disposed at a position substantially corresponding to the lower half of said cathode drum, a copper electrolytic solution is allowed to flow between the cathode drum and the anode, a potential differential is provided between these to electrodeposit copper onto the cathode drum, and the electrodeposited copper is peeled away from the cathode drum when it reaches a specific thickness, so that a copper foil is produced continuously.
A copper foil obtained in this way is generally called a raw foil and, after this, it is subjected to a number of surface treatments and used for printed wiring boards and so forth.
FIG. 1 is a simplified diagram of a conventional apparatus for producing a copper foil. This electrolytic copper foil production apparatus has a cathode drum 1 installed in an electrolysis bath containing an electrolytic solution. This cathode drum 1 is designed to rotate while being partially submerged (substantially the lower half) in the electrolytic solution.
An insoluble anode 2 is provided so as to surround the outer peripheral lower half of this cathode drum 1. A specific gap 3 is maintained between the cathode drum 1 and the anode 2, and an electrolytic solution is allowed to flow through this gap. Two anode plates are disposed in the apparatus shown in FIG. 1.
With the apparatus in FIG. 1, the electrolytic solution is supplied from below, and this electrolytic solution goes through the gap 3 between the cathode drum 1 and the anode 2, overflows from the top edge of the anode 2, and is then recirculated. A rectifier is interposed between the cathode drum 1 and the anode 2 so that a specific voltage can be maintained between the two components.
As the cathode drum 1 rotates, the thickness of the copper electrodeposited from the electrolytic solution increases. When at least a certain thickness is reached, this raw foil 4 is peeled away and continuously taken up. A raw foil produced in this manner is adjusted for thickness by varying the distance between the cathode drum 1 and the anode 2, the flow rate of the supplied electrolytic solution, or the amount of electricity supplied.
A copper foil produced with an electrolytic copper foil producing apparatus such as this has a mirror surface on the side touching the cathode drum, but the opposite side is a rough surface with bumps and pits. Problems encountered with ordinary electrolysis are that the bumps and pits on the rough side are severe, undercutting tends to occur during etching, and fine patterning is difficult.
On the one hand, as the density on printed wiring boards has steadily risen, there has more recently been a need for a copper foil that can be more finely patterned as the circuit width decreases and multilayer circuits are produced. This fine patterning requires a copper foil that has a good etching rate and uniform solubility, that is, a copper foil with excellent etching characteristics.
Meanwhile, the properties required of copper foils for printed wiring boards include not only elongation at room temperature but also elongation properties to prevent cracking due to temperature stress, as well as high tensile stress, to maintain the dimensional stability of the printed wiring board.
However, a copper foil with a highly irregular rough surface is wholly unsuited to fine patterning as described above. Ways are therefore being studied on lowering the profile of the rough surface. It is known that the profile can be lowered by adding large quantities of glue or thiourea to the electrolytic solution.
However, the problem with such additives is that they dramatically lower the elongation percentage, greatly detracting from the foil's properties as a copper foil for printed wiring boards.
2-layer flexible substrates have gained attention as substrates for preparing flexible wiring boards. Because in a 2-layer flexible substrate, a copper conductor layer is provided directly on an insulating film without an adhesive, the substrate itself can advantageously be kept thin and the thickness of the copper conductor layer can be adjusted at will before adhesion. The normal method of manufacturing such a 2-layer flexible substrate is to form an underlying metallic layer by dry plating on the insulating film, and then electroplating copper on top. However, the underlying metallic layer obtained in this way contains numerous pinholes, resulting in exposure of the insulating film and, in the case of a thin copper conductor layer, the areas exposed by the pinholes are not filled in and pinholes occur on the surface of the copper conductor layer, leading to wiring defects. As a means of solving this problem, Patent Document 1, for example, describes a 2-layer flexible substrate manufacturing method in which an underlying metallic layer is formed on an insulating film by a dry plating process, a primary electrolytic copper plating coating layer is formed on the underlying metallic layer and treated with an alkali solution, after which an electroless copper plating coating is adhered and, finally, a secondary electrolytic copper plating coating layer is formed. However, this method involves complex steps.
Patent Document 1: Japanese Patent Publication No. H10-193505